The magnetic properties of chemical substances have been studied extensively. Several types of magnetism are known, including paramagnetism, diamagnetism, and ferromagnetism. A discussion of these magnetic properties is found in Cotton and Wilkinson, Advanced Inorganic Chemistry, Third Ed., 1972, Interscience Publishers, pp. 535-546, the disclosure of which is incorporated herein by reference in its entirety.
A paramagnetic substance is one which is attracted into a magnetic field with a force proportional to the field strength times the field gradient. Paramagnetism is generally caused by the presence in the substance of ions, atoms or molecules having unpaired electrons. Each of these has a definite paramagnetic moment that exists in the absence of an applied magnetic field.
A diamagnetic substance is one which is repelled by a magnetic field. Diamagnetic behavior is due to small magnetic moments induced by an applied magnetic field in opposition to the field. These induced moments do not exist in the absence of the field. All material is diamagnetic to some extent, but the effect is very small and is often masked by the paramagnetic or ferromagnetic effects if the individual molecules have permanent magnetic dipole moments.
A ferromagnetic substance is one which exhibits high magnetic permeability and the ability to acquire magnetization in relatively weak fields, such as iron, nickel, and cobalt.
Magnetic susceptibility is a measurable property of a substance that allows the determination of the magnetic moment of that substance. Magnetic susceptibility is defined as the ratio of the magnetic permeability of a substance to that of a vacuum, minus one. Magnetic susceptibility is positive for paramagnetic and ferromagnetic substances, and negative for diamagnetic substances.
Magnetic permeability is a measure of the ability of a substance to modify a magnetic field, and is equal to the ratio of magnetic induction to magnetic intensity.
Magnetic induction is a vector quantity that specifies the direction and magnitude of magnetic force at every point in a magnetic field.
Magnetic intensity is that part of a magnetic field related solely to external currents as a cause, without reference to the presence of matter.
Magnetic moment is the ratio of the maximum torque exerted on a magnet or electric current loop in a magnetic field to the magnetic induction of the field. Magnetic moment can be calculated from magnetic susceptibility.
Many methods are known for measuring magnetic susceptibility, all of which depend on measuring the force exerted upon a sample when it is placed in an inhomogeneous magnetic field. The more paramagnetic the sample is, the more strongly it will be drawn toward the more intense part of the field; the more diamagnetic the sample is, the more it will be repelled into the weakest part.
A typical prior art method of measuring magnetic susceptibility of a sample involves an apparatus known as a Gouy balance. This apparatus and the method of its use are described in standard texts such as Shoemaker, et al., Experiments in Physical Chemistry, Third Ed., 1974, McGraw-Hill Book Company, pp. 422-434, the disclosure of which is incorporated herein by reference. As disclosed in that reference, such an apparatus can be delicate, expensive, and complex.
It would be desirable to have a method and apparatus to measure quantitative and qualitative changes in the magnetic susceptibility of a sample that avoid the disadvantages of the prior art.
It is thus one object of the invention to provide a simple, relatively inexpensive method and apparatus for determining quantitative and qualitative changes in the magnetic susceptibility of a sample.
It is another object of the invention to provide such a method and apparatus that are adaptable for use in an industrial environment.
It is yet another object of the invention to provide such a method and apparatus that are adaptable for use in non-invasive biomedical applications.
It is still another object of the invention to provide such a method and apparatus that are adaptable for use in non-invasive biomedical applications in a clinical environment.
Other objects of the invention will become apparent from the following description and drawings and the appended claims.